Laplace–fourier-domain full waveform inversion of deep-sea seismic data acquired with limited offsets

Abstract

Laplace-Fourier-domain full waveform inversion is considered one of the most reliable schemes to alleviate the drawbacks of conventional frequency-domain inversion, such as local minima. Using a damped wavefield, we can reduce the possibility of converging to local minima and produce an accurate long-wavelength velocity model. Then, we can obtain final inversion results using high-frequency components and low damping coefficients. However, the imaging area is limited because this scheme uses a damped wavefield that makes the magnitudes of the gradient and residual small in deep areas. Generally, the imaging depth of Laplace-Fourier-domain full waveform inversion is half the streamer length. Thus, dealing with seismic data in the deep-sea layer is difficult. The deep-sea layer reduces the amplitude of signals and acts as an obstacle for computing an exact gradient image. To reduce the water layer's effect, we extrapolated the wavefield with a downward continuation and performed refraction tomography. Then, we performed Laplace-Fourier-domain full waveform inversion using the refraction tomography results as an initial model. After obtaining a final velocity model, we verified the inversion results using Kirchhoff migration. We presented common image gathers and a synthetic seismogram of Sumatra field data to prove the reliability of the velocity model obtained by Laplace-Fourier-domain full waveform inversion. Through the test, we concluded that Laplace-Fourier-domain full waveform inversion with refraction tomography of the downward-continued wavefield recovers the subsurface structures located at depth despite a relatively short streamer length compared to the water depth.